Regio‐, Peri‐, and Torquoselectivity in Hydroxy Heptatrienyl Cation Electrocyclizations: The Iso/Homo‐Nazarov Reaction

Competition: The electrocyclizations of 1‐ and 3‐hydroxyheptatrienyl cations (see scheme) have been computationally studied at the B3LYP/6‐311G(d) level. The 1‐hydroxy system clearly favors a 4e− over a 6e− process, while for the 3‐hydroxy isomer these mechanisms compete. magnified image The electrocyclizations of 1‐ and 3‐hydroxyheptatrienyl cations have been computationally studied at the B3LYP/6‐311G(d) level. The 1‐hydroxy system clearly favors a 4πe− over a 6πe− process, while for the 3‐hydroxy isomer these mechanisms compete. Substituents can either enhance or invert this periselectivity through steric or electronic effects, respectively. Houk's model of torquoselectivity helps to explain the activation energy differences between the alternative 4πe− electrocyclizations available for each system. The cyclopentenyl cations thus obtained can evolve, either through intramolecular trapping by the vinyl group or by 1,3‐proton migration, along reaction coordinates which could correspond to either very asynchronous concerted mechanisms, or two‐step reactions with very shallow or non‐existent intermediates.